System for biometric identity confirmation

ABSTRACT

A biometric identity confirmation system is based on both pulse wave data and spirometric data for the subject. During an initial enrollment mode, pulse wave and spirometric data for a known subject are used to generate subject characterization data for the known subject. During a subsequent identity authentication mode, pulse wave and spirometric data for a test subject are analyzed using the subject characterization data to confirm whether the identity of the test subject matches the known subject.

RELATED APPLICATION

The present application is based on and claims priority to theApplicant's U.S. Provisional Patent Application 61/589,084, entitled“System For Biometric Identity Confirmation,” filed on Jan. 20, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of biometricidentity confirmation. More specifically, the present inventiondiscloses a system for biometric identity confirmation based on analysisof both spirometric data and pulse wave data for a test subject.

2. Background of the Invention

Biometric identification is the process of recognizing or rejecting anunknown person as a particular member of a previously characterized set,based on biological measurements. The ideal biometric characterizationis specific to the individual, difficult to counterfeit, robust tometabolic fluctuations, insensitive to external conditions, easilymeasured, and quickly processed.

Fingerprint, retinal, iris, and facial scans are well-known biometricidentification techniques relying on image processing. Images aretwo-dimensional, requiring sophisticated and computationally intensivealgorithms, the analysis of which is often complicated by randomorientation and variable scaling. Voice recognition is an example ofbiometric identification amenable to time series analysis, an inherentlysimpler one-dimensional process.

The simplest biometric identifiers can be expressed as a singleparameter, such as height or weight. Single parameter identifiers havebeen the only quantitative means of identification throughout most ofhistory. The price of simplicity is the loss of specificity, and in thecase of weight, the lack of constancy over time. Nevertheless,single-parameter biometrics remain effective identifying factors, as isobvious from their continued use.

Identity tracking/confirmation is the process of following thewhereabouts of a known subject moving unpredictably among similarindividuals, perhaps with deceptive intent. Tracking/confirmation issomewhat simpler than identification, because it merely requiresdistinguishing the subject from all others rather than distinguishingevery individual from every other, and because continuous rather thanepisodic data are available. Biometric identity tracking/confirmation isthe continuous verification that a body-mounted sensor has remained onthe subject, and has not been surreptitiously transferred to animpostor. For the purposes of this application, the term “biometricidentification” should be broadly construed to encompass both biometricidentification in its narrower sense, as described above, and identitytracking/confirmation.

SUMMARY OF THE INVENTION

This invention provides a system for biometric identity confirmationbased on both pulse wave data and spirometric data for the test subject.During an initial enrollment mode, pulse wave and spirometric data for aknown subject are used to generate subject characterization data for theknown subject. During a subsequent identity authentication mode, pulsewave and spirometric data for a test subject are analyzed using thesubject characterization data to confirm whether the identity of thetest subject matches the known subject.

These and other advantages, features, and objects of the presentinvention will be more readily understood in view of the followingdetailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction withthe accompanying drawings, in which:

FIG. 1 is a flowchart of the enrollment mode of the present invention.

FIG. 2 is a flowchart of the identity authentication mode of the presentinvention.

FIG. 3 is a flowchart of the “acquire trial” procedure for pulse wavedata.

FIG. 4 is a flowchart of the “acquire trial” procedure for spirometricdata.

FIG. 5 is a flowchart of the procedure used by both the pulse wave andbreath print algorithms to enroll a new client.

FIG. 6 is a flowchart of the identity authentication mode of the presentinvention.

FIG. 7 is a graph showing pulse wave exemplar shape vectors of the tensubjects of a recent study, along with the mean pulse wave shape.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a biometric system for characterizingindividuals by the non-invasive sensing of arterial pulse waves andspirometric data for the purposes of identification and identitytracking/confirmation. The major components include a computerprocessor, data storage, a pulse sensor adjacent to the subject's tissuethat generates time-series data based on the subject's pulse waves, anda spirometric sensor that measures predetermined spirometric propertiesof the exhaled breath sample, such as flow or pressure. This spirometricdata is typically generated as time-series data over the course of thesample.

As an overview, the processor initially receives and analyzes the pulsewave data from the pulse sensor and the spirometric data from thespirometric sensor for a known subject to generate subjectcharacterization data identifying the known subject. Thereafter, in theidentity authentication mode, the processor simultaneously receives datafrom the pulse sensor and spirometric sensor for a test subject (who mayor may not be the known subject). The processor analyzes this data inconjunction with the stored subject characterization data to determinewhether the test subject is the same as the known subject. For thepurposes of this application, it should be understood that the phrase“test subject” refers to the person whose identity is being tested orconfirmed during the identity authentication mode of the present system.

Thus, the present system operates in one of two mutually exclusivemodes—an enrollment mode and an identity authentication mode. Theenrollment mode acquires subject data under the supervision of a trainedtechnician, computes subject characteristics, calculates the probabilityof an impostor producing similar characteristics, and stores thesefindings in a client database for later use during the identityauthentication mode.

FIG. 1 is a general flowchart of the enrollment mode employed toinitially build subject characterization data for a known subject. Theoperator first verifies the identity of the subject (step 20), andmounts and tests the pulse sensor on the subject, and provides thesubject with a spirometric sensor (step 21). The processorsimultaneously acquires pulse wave data from the pulse sensor andspirometric data from the spirometric sensor for a brief period of time(step 22). The subject may be asked to undertake a range of activitiesto ensure the enrollment data are representative of that which may beencountered over the subject's normal day-to-day activities. Theprocessor analyzes the enrollment data and generates subjectcharacterization data for identifying the known subject (step 23). Thissubject characterization data is stored for later use during theidentity authentication mode of the present system (step 24), as will bedescribed below.

The identity authentication mode is used to authenticate the identity ofa test subject, who may or may not be the known subject from theenrollment mode. In this mode, the system acquires subject dataunsupervised in the field, compares it to subject and impostorcharacteristics, and decides whether to authenticate or challengeidentification. FIG. 2 is a flowchart of one possible embodiment of theidentity authentication mode. For each identity authentication test, theprocessor acquires pulse wave data from the pulse sensor and spirometricdata from the spirometric sensor for the test subject (step 25). Theprocessor analyzes this test data using the subject characterizationdata (step 26). Based on this analysis, in step 27, the processordetermines whether there is a sufficient degree of similarity betweenthe pulse wave and spirometric characteristics of the known subject(from the subject characterization data) and the test subject toconclude that these subjects are the same person (step 28). If so, theprocessor may update the subject characterization data 18 to include thecurrent test data (step 28A) and then loop back to step 25. Otherwise,if the processor determines that the current test subject is not thesame as the known subject, an alarm can be activated to signal thatdeception has been detected (step 29).

As will be discussed below, the two modes in the preferred embodiment ofthe present invention share a common “acquire trial” procedure thatacquires and pre-processes a short, contiguous time-series data of thedigitized measurement, called a “trial”.

FIG. 3 shows how the pulse wave algorithm acquires a trial. The trialpulse wave typically consists of a small number (e.g. 10) of pulsecycles, which are similar but not identical to each other. Performanceis improved by synchronizing and summing pulse cycles to wash out noise.The goal of the procedure is to convert the multi-cycle waveform into asingle representative cycle. Block 300 is the start of the procedure.Block 301 reads and discards pulse samples for a fixed duration (e.g. 2seconds) while the waveform settles. After settling, block 302 reads andrecords samples for the remainder of the trial (e.g. 8 seconds). Theconcurrently running block 303 tests samples for “railing” (i.e.exceeding the limits of the digitizer, an indicator of trialcorruption). Upon detecting a railed sample, Block 303 calls block 304,which rejects the trial and stops the procedure. If no recorded samplesare railed, control proceeds to block 305, which calculates the firstand second derivatives of the pulse wave with respect to time, toeliminate baseline drift and generate triggers associated with thesystolic excursion. Representing the subject's pulse wave with its firstderivative also obscures the bio-informational nature of the signal,thus enhancing privacy. The derivatives may or may not be smoothed toreduce high frequency noise. Block 306 chooses the most negativeexcursion of the second derivative as the “trigger candidate” (TC).Next, block 307 zeroes the TC and some small number (e.g. 4) ofimmediate predecessor and successor data, to avoid selecting the samepeak again. Then, block 308 compares the present TC to the first TC. Ifthe present TC is greater than some threshold fraction (e.g. ½) of thefirst TC, the procedure loops through blocks 306-308, acquiring anotherTC. If not, TC acquisition is deemed complete, and control proceeds toblock 309. If there are many (e.g., 8) more TCs than can be accountedfor according to the settled sampling time and maximum pulse rate (e.g.,16 for 8 seconds at 120 beats per minute), the sample is judged toonoisy, and block 309 calls block 304, which rejects the trial and stopsthe procedure. If not, the trial is accepted, but some of the TCs may benoise spikes asynchronous to the underlying pulse cycle.

The section comprising blocks 310-316 is called the “trigger sieve”because it removes asynchronous false triggers, thus enhancingperformance (recent data showed useable trial yield increasing from ⅕ to⅘). Block 310 calculates a square matrix of the delays Δ between everypair of TCs. Next, the procedure loops through all integer pulseperiods, in units of the sampling period, from the fastest to theslowest measurable pulse (e.g., 50-150 for 100 Hz sampling and 120-40beats per minute), to find the best fit to the preponderance of TCs.Block 311 increments the pulse period P. Block 312 computes the matrixof squared remainders [Δ mod P]², where the “mod” operation yields theinteger remainder with the smallest absolute value (e.g., 15 mod 8equals −1, not 7). Block 313 sums the squared remainders for each TCrelative to the other TCs, and normalizes such that a “score” near (muchsmaller than) unity indicates P is a poor (good) fit to the true pulseperiod. Block 314 averages the TC scores to evaluate P's goodness offit. Block 315 selects the P with the lowest score. Next, block 316rejects TCs with optimal-P scores greater than a preset threshold (e.g.,0.6) as false triggers not synchronized with the prevailing pulsatilerhythm, leaving the remaining TCs to serve as true triggers. Then, block317 uses the true triggers to synchronize and sum the cycles. Finally,block 318 returns the summed cycle to the calling program.

FIG. 4 shows how the breath print algorithm acquires a trial. The trialconsists of a single forced exhalation, preceded by a quiescent periodused to establish a signal baseline, and succeeded by a period used toensure further exhalation is not forthcoming to spoil the data. Thegoals of the procedure are to delineate these three periods, subtractthe baseline from the exhalation, measure the exhalation duration andforced vital capacity (FVC), normalize the exhalation shape to duration,and compute the auxiliary parameters forced expiratory volume in theinitial second (FEV₁) and peak expiratory flow (PEF). Block 400 is thestart of the procedure. Block 401 initiates a data acquisition andprocessing loop that repeats for a fixed number of trial samples (e.g.1500 for a 15 second trial digitized at 100 Hz). Blocks 402 and 403repeatedly acquire samples until the flow signal exceeds a thresholddefining the onset of exhalation. Block 404 determines if there aresufficient samples to establish a baseline (e.g. 50): If not, controlpasses to block 405, which rejects the trial and stops the procedure. Ifso, the average of the baseline samples is recorded and control passesto block 406. Blocks 406 and 407 repeatedly acquire samples until theflow signal falls below the previously defined threshold, at which timeexhalation is deemed to have ceased. Block 408 determines if there aresufficient exhalation samples (e.g. 250): If not, control passes toblock 405, which rejects the trial and stops the procedure. If so, thenet (baseline-subtracted) exhalation samples are recorded and controlpasses to block 409. Blocks 409 and 410 acquire post-breath samplesuntil the trial times out, or a sample exceeds the previously definedthreshold, indicating a resurgence of exhalation. In the latter case,the trial is deemed corrupted, and control passes to block 405, whichrejects the trial and stops the procedure. In the former case, block 411passes control to block 412, which determines if there are sufficientpost-breath samples (e.g. 1). If not, control passes to block 405, whichrejects the trial and stops the procedure. If so, control passes toblock 413, which computes the exhalation duration and FVC. Block 414uses these to normalize the exhalation shape in time and amplitude, forcomparison to other exhalations of different durations and FVCs. Block415 computes the FEV₁ and PEF. Block 416 returns duration, FVC, shape,FEV₁ and PEF to the calling routine, and ends the procedure.

FIG. 5 shows two embodiments of the procedure used by both the pulsewave and breath print algorithms to enroll a new client. This procedurecan be used both to establish the client's characteristics as a subjectwhose identity will be putative in the field, and as a possible impostorfor any other client. Block 500 is the start of the procedure. Block 501acquires a number of trials (e.g., five) by repeatedly calling theappropriate Acquire Trial procedure. Block 502 computes the “exemplar”(i.e., the arithmetic mean over the enrollment trials of any or all ofthe pulse wave shape vector, the breath print duration and FVC scalars,the breath print shape vector, and the breath print FEV₁ and PEFscalars, arranged into a vector) using the enrollment trials. Block 503computes the statistics (i.e., the covariance matrix) of the enrollmenttrial, as well as the relative weights of the shape vector components.The latter may incorporate either or both of two independentinnovations: dynamic weighting, in which portions of the shape vectorthat are more repeatable from trial to trial are accentuated relative toless repeatable portions; and feature weighting, in which portions ofthe shape vector that are more specific to the subject are accentuatedrelative to portions more typical of the population at large.

Block 504 transfers control to one of two blocks, depending on whetherthe “fixed authentication threshold” or the “Bayesian optimal decision”embodiment of the algorithm is selected. The chief distinction is thatthe Bayesian embodiment makes use of potential impostor data (i.e., fromother clients as potentials impostors for the subject), while the fixedthreshold does not. Block 505 finds the principle components of thecovariance matrix, and uses the dominant eigenvector (i.e., that withthe largest eigenvalue) to linearly combine the parameter vector into ascalar “composite parameter”, which is optimal in the sense that theenrollment data has the greatest correlation, and thus the least spread,along the dominant eigenvector. In general, this results in unequalweighting of the parameters in the decision to authenticate or challengeidentity. Next, block 506 computes the authentication thresholdcorresponding to the preset desired true authentication probability(e.g., ⅞). Then, block 507 enrolls the client, and block 508 stops theprocedure. On the other bifurcation, block 509 expands the ratio of thesubject probability density to the impostor probability density tosecond order in the deviation from the subject exemplar. Block 510includes the effects of the generally unequal penalties of falseauthentication and false challenge, and the a priori probability ofattempted deception, which varies among clients. Since the Bayesianoptimal decision embodiment uses the entire covariance matrix, it is notnecessary or advantageous to define a composite parameter; and sinceimpostor data is incorporated, the true and false authenticationprobabilities can be traded.

FIG. 6 shows how either algorithm embodiment decides whether toauthenticate or challenge the subject's identity based on a field trial.Block 600 is the start of the procedure. Block 601 acquires a fieldtrial, and block 602 subtracts the subject exemplar to yield the“deviation”, a vector with the same structure as a trial, and optionallyapplies dynamic or feature weighting to the deviations of the shapevectors. Block 603 transfers control to one of two blocks, depending onwhether the “fixed authentication threshold” or the “Bayesian optimaldecision” embodiment of the algorithm is selected. Block 604 computesthe optimal composite parameter for the deviation, and block 605compares it to the authentication threshold. If greater, block 606advises authorities to authenticate the subject's identity, and block607 stops the procedure. If lesser, block 608 advises authorities tochallenge the subject's identity. On the other bifurcation, block 609computes the ratio of the subject probability density to the impostorprobability density to second order in the deviation of the field trialfrom the subject exemplar, and block 610 compares it to zero. Ifpositive, block 606 advises authorities to authenticate the subject'sidentity. If negative, block 608 advises authorities to challenge thesubject's identity.

As so far described, the algorithm uniformly weights each exemplar shapevector component, placing equal importance on the various features.However, this restriction is unnecessary, and may not be optimal. Someparts of some subjects' exemplars are more characteristic than otherparts, so it's reasonable to suppose weighting unusual features moreheavily could enhance the distinguishability of subjects.

FIG. 7 shows the pulse wave exemplar shape vectors of the ten subjectsof a recent study, along with the mean pulse wave shape. Generally, somesubjects are more atypical than others, and therefore are more easilyidentified in the field. Some subjects have features (e.g., subject 26MJB near 0.27 seconds) that are quite distinctive. If these features areweighted more heavily than more typical regions (e.g., subject 26 MJBnear 0.14 seconds), the subject is more readily recognized whensupplying a legitimate field trial, and less easily mimicked by animpostor. An example feature-weighting strategy is to weight each fieldtrial shape vector component proportionally to the square of thedeviation of the corresponding subject exemplar component from the meanexemplar component, thus placing greater weight on more unusualfeatures.

One technique for implementing dynamic weighting is to parse the shapevector into segments that are large enough to avoid excessivestatistical fluctuations, yet small enough to provide resolution of thevarying character across the vector (e.g., a 100-component breath printvector into 20 five-component segments), and assign a different weightto each segment based on its fluctuations. An example dynamic-weightingstrategy is to weight each field trial shape vector segmentproportionally to the reciprocal of the segment's variance (i.e., thesum over enrollment trials and segment components of the squareddeviation of the enrollment trial component from the exemplarcomponent), thus placing greater weight on more repeatable segments.

One technique for implementing feature weighting is to raise each shapevector component probability to a different power greater or less thanunity, according to how much the exemplar shape deviates from theaverage subject at that point. The feature weighting function can beexpressed as a vector of the same dimensionality as the shape itself,consisting of components whose average is unity (equal weighting isencompassed as the special case where all components are 1). Thisapproach keeps the rest of the algorithm unaffected by whether featureweighting is selected or disabled. In general, the feature weightingvector is different for each client.

The above disclosure sets forth a number of embodiments of the presentinvention described in detail with respect to the accompanying drawings.Those skilled in this art will appreciate that various changes,modifications, other structural arrangements, and other embodimentscould be practiced under the teachings of the present invention withoutdeparting from the scope of this invention as set forth in the followingclaims.

We claim:
 1. A method for biometric identity confirmation of a subjecthaving a pulse and a respiratory cycle, said method comprising: duringan initial training mode, simultaneously acquiring pulse wave shape dataand spirometric data from a known subject; generating and storingsubject characterization data for the known subject derived at least inpart from both the pulse wave shape data and spirometric data for theknown subject; and during a subsequent identity authentication mode,simultaneously acquiring pulse wave shape data and spirometric data froma test subject, and analyzing the pulse wave shape data and spirometricdata with the subject characterization data for the known subject toconfirm whether the identity of the test subject matches the knownsubject; wherein the step of generating subject characterization datafurther comprises use of a second derivative of the pulse wave shapedata with respect to time to identify pulse cycle start points forsynchronous averaging of multiple pulse cycles to create an exemplar forthe known subject.
 2. The method of claim 1 wherein the subjectcharacterization data is derived at least in part from first derivativeof the pulse wave shape data with respect to time.
 3. The method ofclaim 1 wherein the second derivative of the pulse wave shape data isused to identity trigger candidates for the pulse cycle start point; andfurther comprising an analysis of a time delay between triggercandidates to thereby discard false trigger candidates and identify truetrigger candidates for synchronous averaging of multiple pulse cycles.4. The method of claim 1 wherein the step of generating subjectcharacterization data further comprises: computing an exemplar in theform of a parameter vector from the pulse wave shape data andspirometric data for the known subject; computing a covariance matrixfrom the pulse wave shape data and spirometric data for the knownsubject; computing an optimal composite parameter from the covariancematrix and parameter vector that is characteristic of the known subject;and computing an authentication threshold corresponding to a desiredtrue authentication probability for the known subject.
 5. The method ofclaim 4 wherein the step of analyzing the pulse wave shape data andspirometric data with the subject characterization data for the knownsubject to confirm whether the identity of the test subject matches theknown subject further comprises: computing a deviation of the pulse waveshape data and spirometric data for the test subject from the exemplarfor the known subject; computing an optimal composite parameter from thedeviation; and confirming the identity of the test subject matches theknown subject if optimal composite parameter is greater than theauthentication threshold for the known subject.
 6. The method of claim 1wherein the step of generating subject characterization data furthercomprises: computing an exemplar in the form of a parameter vector fromthe pulse wave shape data and spirometric data for the known subject;computing a covariance matrix from the pulse wave shape data andspirometric data for the known subject; and computing a probabilitydistribution ratio of a weighted subject/impostor probability density bya Bayesian optimal decision analysis of the parameter vector, covariancematrix, and data from other subjects as potential impostors for theknown subject.
 7. The method of claim 6 wherein the step of analyzingthe pulse wave shape data and spirometric data with the subjectcharacterization data for the known subject to confirm whether theidentity of the test subject matches the known subject furthercomprises: computing a deviation of the pulse wave shape data andspirometric data for the test subject from the exemplar for the knownsubject; computing a weighted subject/impostor probability density ratiofor the deviation; and confirming the identity of the test subjectmatches the known subject if the weighted subject/impostor probabilitydensity ratio is greater than zero.
 8. The method of claim 1 wherein thestep of generating subject characterization data further comprises:computing an exemplar from the pulse wave shape data by synchronousaveraging of multiple pulse cycles; and weighting portions of theexemplar selected based on their repeatability observed during theinitial training mode.
 9. The method of claim 1 wherein the step ofgenerating subject characterization data further comprises: computing anexemplar from the pulse wave shape data by synchronous averaging ofmultiple pulse cycles; and weighting portions of the exemplar selectedto distinguish characteristic features of the known subject observedduring the initial training mode.
 10. A method of claim 1 wherein thestep of generating subject characterization data for the known subjectfurther comprises measurement of characteristic changes in the pulsewave shape data as a function of the phase of the respiratory cycle ofthe known subject.